Writing and reading data requires the data medium to be illuminated with a light spot produced by accurately focussing incident radiation. Pre-formatting patterns are disposed on the reference surface of the data medium in order to distinguish adjacent track elements. The formatting patterns are situated outside the areas which are reserved for the data.
Each track element is thus split up into alternating sites which are scanned in succession by the light spot.
When the light spot is being used for reading the pre-formatting patterns and the contents of the data blocks, the data medium must be illuminated in such a manner as to avoid changing its optical characteristics, while still being sufficiently brightly illuminated to ensure that the detected signals are accurate. In contrast, when data is being recorded, the energy received by the data medium is increased in order to effect proper recording thereof. In this type of operation at two levels of brightness, the high brightness level is strictly limited to the data areas since it is important to avoid damaging the pre-recorded formatting patterns. To this end, the read/write device is provided with a brightness control circuit for controlling the power of the incident radiation and for taking account of the alternation between the special formatting areas having the pre-recorded formatting patterns thereon and the larger data areas in which data may be written.
The use of an optical memory necessarily includes both writing and reading stages, however in most types of use, both of these stages occupy only a very small proportion of the total operating time. Thus, when using an optical disk memory, the disk is kept rotating and the focusing, synchronizing, and tracking functions are all kept operating even when no read or write operation is to be performed. This standby state ensures that the memory responds rapidly when suitable instructions are received The density of energy on the illuminated portion of the disk is high while scanning the pre-formatted patterns, since the spot must read these patterns with great accuracy. The same is true when reading or writing data, however it will readily be understood that in the standby state, or when performing a function other than writing or reading, it is possible to modify the density of illumination which is received by the medium.
Optical memories, and in particular "reversible", i.e, erasable and re-writable memories are subject to aging like any other medium, and it is well know that aging is accelerated by all kinds of stresses. It is also well known that in order to erase a disturbance of given size, selective means for performing erasure must act over an area which is at least slightly larger than the area in which writing has taken place, and that such erasure means must not reproduce the same conditions that occur during writing.
Given the above conditions, and ignoring the writing of data, it may be considered to be a drawback to maintain the illumination conditions of a data medium constant during the remainder of its operating time.
Preferred embodiments of the present invention mitigate this drawback by providing a common illumination mode of the data medium for reading transcribed data or for reading the pre-formatted patterns which lie between the areas reserved for storing data, and proposes modifying the illumination mode while scanning these areas during a stage which is neither a write stage nor a read stage.